1. Field of the Invention
The present invention relates to a signal converter for converting digital input signals to optical modulation signals to output the converted signals, and particularly to a signal converter wherein amplitude control and bias control are simplified.
2. Description of the Related Art
A well-known optical modulator (for example, an LN modulator) is used as a means for optical modulation in a signal converter in which electrical digital signals are converted to optical modulation signals. This type of an optical modulator applies intensity modulation to continuous light having constant intensity. There are a signal electric power proportional to digital input signals and a bias electric power composed of direct current, as an electric power for intensity modulation to be applied to the continuous light. Such signal electric power and bias electric power must be appropriate values for optical modulation signals to be linear with respect to digital input signals without signal distortion.
FIG. 1 shows a conventional signal converter including a circuit for obtaining suitable values of signal electric power and bias electric power, respectively. Namely, the signal converter of FIG. 1 includes a light source 1 for emitting a continuous light of a constant intensity, an optical modulator 2, a driver 3 for amplifying a digital input signal to supply a signal electric power, an oscillator 4 for generating a pilot signal of a frequency f, a DC voltage source 5 for generating a DC voltage in response to feedback of an amplitude monitor signal from an output (or an amplitude monitor terminal) of the driver 3, an adder 6 for superimposing the DC voltage on the pilot signal to supply an output signal to an amplitude control terminal of the driver 3, a splitter 7 for providing a part of optical modulation signals to be output from the optical modulator 2, a monitor photodiode (monitor PD) 8 for receiving a part of the optical modulation signals, a monitor amplifier 9 for amplifying a received electric power of the monitor PD 8 to obtain a monitor signal, a mixer 10 for multiplying the monitor signal by the pilot signal, a low pass filter 11 for removing a high-frequency component (AC component) from the multiplied signal to provide a low-frequency component (DC component) as a monitor phase signal, a differential amplifier 12 for controlling a bias electric power in such that the monitor phase signal reaches a GND level, the monitor phase signal being input to the differential amplifier 12 wherein the other input is connected to GND, and a modulation electric power applier (bias T) 13 for superimposing the bias electric power output from the differential amplifier 12 on an AC component of the signal electric power output from the driver 3 to apply an output signal to the optical modulator 2.
As shown in FIG. 1, a feedback system 14 for amplitude control is constituted by a loop of the driver 3, the DC voltage source 5, and the adder 6. Furthermore, a monitor circuit 15 for receiving a part of optical modulation signals output to provide it as a monitor signal includes the splitter 7, the monitor PD 8, and the monitor amplifier 9. Moreover, a feedback system 16 for bias control is constituted by a loop of the differential amplifier 12, the bias T, the optical modulator 2, the monitor circuit 15, the mixer 10, and the low pass filter 11.
In the signal converter shown in FIG. 1, a pilot signal of a frequency f is input to an amplitude control terminal of the driver 3 amplifying digital input signals, whereby amplitude of a signal electric power to be supplied to the optical modulator 2 is controlled. A DC voltage of the DC voltage source 5 is controlled in such that an amplitude monitor signal of the driver 3 becomes constant in the feedback system 14, whereby a signal electric power is controlled. However, it is necessary for adjusting initially elements contained in the feedback system 14 in order to obtain the optimum amplitude in the optical modulator 2.
To control automatically a bias electric power at the optimum point, a part of optical modulation signals output from the optical modulator 2 is provided by the splitter 7, and it is received by the monitor PD 8. The received electric power is amplified by the monitor amplifier 9 to obtain a monitor signal, the monitor signal is multiplied by a pilot signal in the mixer 10, and a monitor phase signal being a DC component is provided by the low pass filter 11. When the monitor phase signal is input to the differential amplifier 12 in which the other input is connected to GND, a bias electric power is automatically controlled in such that the monitor phase signal reaches the GND level (Japanese patent No. 2642499).
In a conventional signal converter, it is required to adjust initially elements contained in the feedback system 14 in order to obtain the optimum amplitude in the optical modulator 2. The initial adjustment is carried out by such a manner that operations for exchanging parts of the feedback system 14 or operations for adjusting finger grips of the parts of the feedback system 14 are repeated while measuring optical outputs of the signal converter by means of an external measuring device. As a result, such initial adjustment decreases mass productivity of signal converter, besides it is an obstruction for reducing a cost therefor.